1. Technical Field
The present specification describes a power supply device, and more particularly, a power supply device using a synchronous rectifier step-down converter.
2. Discussion of the Background
A source voltage required by a power amplifier used for a cell phone or the like depends on output power of the power amplifier, and ranges from about 0.6 V to about 3.5 V.
For example, when a power amplifier requires a source voltage of 1 V, if a battery voltage of about 3.5 V is used, the power amplifier consumes more electrical power than necessary. Therefore, in order to supply a circuit driven at a voltage smaller than the battery voltage with a source voltage smaller than the battery voltage, conventional power supply devices use a step-down DC-DC converter such as a switching regulator.
However, such DC-DC converters themselves consume power due to their use of inductor and switching elements. Therefore, when the power supply device does not need to step down a battery voltage as an input voltage, the power supply device stops operation of the switching regulator and outputs the input voltage as is by bypassing the step-down DC-DC converter using a bypass circuit.
However, once the source voltage outputs the input voltage by using the bypass circuit bypassing the step-down DC-DC converter, when the power supply device resumes step-down operation of the step-down DC-DC converter, switching operation resumes in a state in which there is a high voltage across an output terminal of the step-down DC-DC converter. As a result, a synchronous rectifier switch is rapidly turned on, resulting in overshoot and ringing, making the output voltage unstable.
In order to address that problem, one related-art power supply device includes a synchronous rectifier step-down converter, a voltage generator, a regulator, and a pulse-width modulator. The synchronous rectifier step-down converter alternately turns on and off a switch and a synchronous rectifier switch. The voltage generator outputs a voltage greater than that of the step-down converter. The regulator outputs an error voltage such that an output voltage of the step-down converter approaches a predetermined reference voltage. The power supply device chooses either the step-down converter or the voltage generator to output a desired voltage. When the voltage generator is chosen, the regulator offsets the error voltage such that the synchronous rectifier switch is turned off.
Since the error voltage of the regulator is offset when an output voltage of the voltage generator switches to a voltage of the step-down converter, the synchronous rectifier switch starts switching operation after an off-state, thereby preventing the synchronous rectifier switch from being turned on for a long period of time. As a result, the power supply device supplies a stable output voltage with reduced overshoot.
However, since the regulator adds an offset voltage from an offset circuit to the error voltage and supplies the output to the pulse-width modulator, it takes a long time until the output voltage reaches a predetermined voltage, resulting in decreased responsiveness.
Accordingly, there is a need for a technology to provide a power supply device capable of stably generating an output voltage with increased responsiveness.